1. Field of the Invention
The invention relates to non-return Faraday effect systems.
2. Description of the Prior Art
Non-return Faraday effect system designates an optical device arranged so that a light beam having crossed it in one direction may not cross it again in the other direction. A known example of such a device is shown diagrammatically in FIG. 1. It comprises, aligned on a same optical axis 1, a polarizer 2, an element 3 making the polarization of a beam rotate, designated hereinafter as a rotator and an analyzer 4.
A non-polarized light beam 10 crosses the polarizer; light polarized in a direction shown by the vector D1 emerges therefrom. The light crosses the rotator, which comprises a bar 3 subjected to a longitudinal magnetic field H; the direction of polarization of the light emerging from the rotator is shown by the vector D2; it has rotated through an angle a proportional to the value of the magnetic field H and to the length of the bar 3 (Faraday effect). That angle a is chosen equal to 45.degree.. The analyzer 4 is set in such a direction that its plane of polarization be parallel to the vector D2. Thus, it allows the light beam emerging from the rotator to pass. If the light beam emerging from the analyzer attempted to pass through again, it would be stopped; indeed, the polarization vector of the returning light may be decomposed into a component perpendicular to D2 and a component parallel to D2. The perpendicular component is stopped by the analyzer 4; the parallel component is stopped by the analyzer, for it undergoes, in the rotator, a rotation of 45.degree. which thus brings it to 90.degree. from the analyzer.
Such a device operates properly. But it should be observed that a device of this kind entails, for the light beam, the crossing of an appreciable thickness of glass (several tens of centimeters).
In a high power glass laser, comprising a generator and several amplifiers, the beam crosses a great length of glass (1 to 3 meters).
It is known that the light beam has a tendency to become self-focussed, more particularly because of the variation in index of the glass due to the presence of intense electric fields.
The longer the portion of glass crossed is, the greater the focussing of the beam is and the more there are risks of damaging the device. That is why it is imperative to try to reduce, as much as possible, the length of glass crossed by the beam.
The applicant has thus sought to produce a non-return device in which the length of glass crossed by the beam is as slight as possible, so as to reduce the self-focussing effect of the beam.
The aim of the invention is therefore to produce a non-return optical device based on the principle set forth above and in which the length of glass crossed by the beam is slight. Another aim of the invention is to combine in a single device such a non-return optical device and a laser amplifier.
The invention has as its object a non-return optical system, characterized in that it comprises a number N of plates having parallel faces made of a material having a Verdet constant and means for applying a magnetic field to the plates as a whole, the first plate of the assembly which receives the incident light beam, being directed, in relation to that beam, so that the angle of incidence be close to the Brewster angle, the planes of the various other plates, each being set in such a direction that on the one hand, the angle of incidence of the beam received on a plate be close to the Brewster angle, and that, on the other hand, the angle between the normal to the plane of that plate and the normal to the plane of the plate situated upstream therefrom, be equal to the angle through which the vector of polarization of the light on crossing the upstream plate has been rotated because of the Faraday effect, and within the number of plates, the material of which they are made, their thickness and the value of the magnetic field being chosen so that the total rotation undergone by the polarization vector of the light between the input of the system and the output be approximtely 45.degree..
The invention also has as its object a laser amplifier characterized in that it consists of a non-return optical device as described hereinabove, in which the material of the plates is an active material such as glass doped with neodymium, the device being, moreover, provided with a means for exciting the active material, such as a flash.
The invention will be well understood from the description given hereinafter of several examples of embodiment of the invention, with reference to the accompanying drawings.